The present application claims priority to Japanese Application No. P11-035086 filed Feb. 12, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a multi-layered optical disc having its recording unit comprised of plural information recording layers. More particularly, it relates to a multi-layered optical disc having its information recording layer formed of a phase change material.
2. Description of Prior Art
Recently, as so-called multi-media are becoming popular, a demand is raised for an optical recording medium handling a large volume of the information, such as a digital moving picture, such that the necessity of storing the large volume of the information and having a random access thereto as necessary for recording and/or reproduction is increasing.
Among the random-accessible recording mediums, there is an optical recording medium of a large storage capacity which is removable, that is can be taken out from the recording and/or reproducing apparatus. This optical recording medium is used in large quantities in many fields.
Under these circumstances, an optical recording medium on one surface of which recording and/or reproduction for four hours is possible in accordance with the NTSC (National Television System Committee) system.
In this optical recording medium, it is targeted to enable recording and/or reproduction for four hours, as a video disc player for home use, to provide a function as a new recording medium which takes the place of the video tape recorder now in prevalent use. Moreover, the optical recording medium may be of the same shape and size as the digital audio disc, having the music data recorded thereon, to prove a product more benign to a user accustomed to a digital audio disc.
Meanwhile, a demand is raised for an optical recording medium capable of handling a still larger information volume, such that the optical recording medium is required to have a still larger storage volume.
For example, the optical recording medium, capable of recording and/or reproduction in accordance with the NTSC system, is required not only to be in a disc shape to exploit the high accessing speed proper to the disc shape to provide a simple and small-sized recording medium, but also to be of a large storage capacity to have multiple functions such as instant reproduction of a recorded picture, trick play or editing.
In order to have these variegated functions, the capacity not less than 8 GB is required. However, there lacks up to now an optical recording medium capable of realizing this large storage capacity by the following reason:
In the already proposed replay-only DVD (digital versatile disc), the wavelength xcex is 0.65 xcexcm, with the numerical aperture NA of the optical system being 0.6, such that the recording capacity possible with the current DVD is not above 4.7 GB.
Thus, in order to provide a recording capacity not less than 8 GB, with the signal format of the DVD system, such as ECC (error correction code) or the modulation system, remaining unchanged, the following relation:
4.7xc3x97(0.65/0.60xc3x97NA/xcex)2xe2x89xa78xe2x80x83xe2x80x83(1)
needs to be met.
From the equation (1), it is necessary that NA/xcexxe2x89xa71.20, such that a shorter wavelength or a larger magnitude of NA is required.
If the NA is of a larger magnitude, a transparent substrate of the optical disc, through which the illuminated reproducing light is transmitted, needs to be reduced in thickness. The reason is that, as NA is increased, the allowance for the aberration ascribable to an angle by which the disc surface deviates from the vertical with respect to an optical axis of the optical pickup, or the so-called tilt angle, becomes smaller, with the aberration ascribable to this tilt angle increasing with the increased thickness of the transparent substrate through which is transmitted the reproducing light.
By the same reason, variations in the thickness of the transparent substrate, through which is transmitted the reproducing light, need to be comprised within a pre-set range.
Meanwhile, an injection molded substrate, formed of plastics, is prevalently used as the transparent substrate for the optical recording medium. This injection molded substrate is difficult to manufacture with an extremely thin thickness and to a high accuracy.
On the other hand, the recording density of the information recording layer of an optical recording medium in an in-plane direction, that is in a two-dimensional direction, is determined by the minimum spot diameter of the laser light in use. Thus, the smaller the minimum spot diameter, the higher may be the density with which the signals are recorded. Therefore, in order to enable high density recording on an optical recording medium, it is attempted to reduce the wavelength of the light source and to increase the numerical aperture NA of the objective lens to reduce the minimum spot diameter. However, there is imposed a technical limitation in shortening the wavelength of the laser light or in increasing the numerical aperture NA of an objective lens, such that increasing the recording density in the two-dimensional direction is approaching its limit.
As means for increasing the recording capacity in an optical disc, there are such method as increasing the number of the recording layers, in addition to the methods of further reducing the recording and/or reproducing wavelength or improving the light converging power (NA) of the light used for recording and/or reproduction. In the write-once or overwrite optical disc, the preference is towards reducing the recording and/or reproducing wavelength and towards a higher NA of the objective lens, whilst multi-layered optical disc was first introduced in Society Publications only in October 1998.
The purport of this report was that recording and/or reproduction is possible in both the two phase change recording layers. It is however feared whether or not, if the information has been recorded on the light incident side recording layer (first recording layer), no information has been recorded on the first recording layer or if a recorded region and a non-recorded region co-exist in the first recording layer, recording can be made in stability in the other recording layer (second recording layer). In actuality, with the phase change recording material, it has been demonstrated that light transmittance of the first recording layer is higher in the recorded state, that is in the amorphous state, than in the non-recorded state, that is in the crystalline state, of the recording material. According to the reports, the light transmittance of the first recording layer is 70% and 45% in the crystalline state and in the amorphous state, respectively.
If the light transmittance of the first recording layer is varied appreciably before and after recording on this layer, it is necessary to record the information on the second recording layer after recording the information on the entire surface of the first recording layer. However, since signals cannot be recorded nor reproduced freely on each of the first and second recording layers, there are imposed severe limitations on the information exchange with the media, with the result that the merit proper to the multi-layered optical disc tends to be lost. Moreover, the transfer rate of the information recorded or reproduced on or from an optical disc recently is desired to be increased, so that an increasing demand is raised for simultaneously recording and/or reproducing plural recording layers of the multi-layered optical disc to increase the transfer rate by a factor corresponding to the number of the recording layers.
Moreover, if, in a recording and/or reproducing optical disc having layered information recording layers, the linear speed in raised by reducing the beam spot diameter, difficulties may be encountered in correctly recording the data, so that, if two information recording layers are layered, it has been difficult to realize the recording capacity not lower than 10 GB.
As discussed above, in an optical recording medium, further increase in the recording capacity represents one of incumbent technical tasks.
It is therefore an object of the present invention to provide a multi-layered optical disc in which the recording density can be increased in the three-dimensional direction and the recording capacity can be increased further and which is superior in recording and/or reproducing characteristics.
It is another object of the present invention to provide a multi-layered optical disc in which recording and/or reproduction can be performed simultaneously in stability in the respective recording layers of the recordable multi-layered optical disc.
It is yet another object of the present invention to provide a multi-layered optical disc which allows for rewriting and high density recording.
In one aspect, the present invention provides a multi-layered optical disc in which two or more information recording layers are layered on a substrate 0.3 to 1.2 mm in thickness, with the interposition of a transparent layer, to form a recording unit, a light transmitting protective layer is formed on the recording unit to a thickness of 10 to 177 xcexcm, and in which the light is illuminated from the side of the light transmitting protective layer to effect recording and/or reproduction of information signals, wherein at least one of the information recording layers except the information recording layer formed at a remotest position from the light transmitting protective layer has a phase change material as a recording material, and wherein a refractive index nc1 and an extinction coefficient kc1 in the crystallized state and refractive index na1 and an extinction coefficient ka1 in the amorphous state of the phase change material constituting the first information recording layer as counted from the light transmitting protective layer satisfy the relationship of (na1/nc1)xe2x89xa612, (kc1/ka1)xe2x89xa612 and (kc1/ka1)xe2x89xa65 (nc1/na1), where the cases of kc1/ka1 less than 1 and nc1/na1 less than 1 are excluded.
In the above-described multi-layered optical disc, according to the present invention, in which the information recording layer is multi-layered, the recording density is increased even in the three-dimensional direction corresponding to the direction of thickness of the information recording layer, thus further increasing the recording capacity.
Moreover, in the multi-layered optical disc according to the present invention, at least one of the information recording layers except the information recording layer formed at a remotest position from the light transmitting protective layer has a phase change material as a recording material. Thus, the multi-layered optical disc of the present invention is able to record and/or reproduce the information repeatedly, whilst it is able to have plural layers capable of recording and/or reproduction.
Also, since the multi-layered optical disc according to the present invention is optically optimized with respect to the light with the wavelength of 380 to 450 nm, it is possible to realize a larger recording capacity.
In another aspect, the present invention provides a multi-layered optical disc in which two or more information recording layers are layered on a substrate 0.3 to 1.2 mm in thickness, with the interposition of a transparent layer, to form a recording unit, a light transmitting protective layer is formed on the recording unit to a thickness of 10 to 177 xcexcm, and in which the light is illuminated from the side of the light transmitting protective layer to effect recording and/or reproduction of information signals, wherein each information recording layer has a phase change material as a recording material, and wherein, if, when the recording and/or reproducing light is converged on the second recording layer remote from the light transmitting protective layer, there co-exist an information recorded track and a non-information recorded track in the recording and/or reproducing light passage area in the first recording layer on the side of the light transmitting protective layer, the phase change recording material of the first recording layer satisfies the following relationship:
0.8xe2x89xa6T1c/T1axe2x89xa61.2
70%xe2x89xa7T1cxe2x89xa740%
70%xe2x89xa7T1axe2x89xa740%
xe2x80x83A1c/A1axe2x89xa71.0
where T1c is the light transmittance of the phase change recording material of the first recording layer in the crystal state, A1c is the absorption rate of light absorbed by the phase change recording material of the first recording layer in the crystal state, T1a is the light transmittance of the phase change recording material in the amorphous state and A1a is the absorption rate of light absorbed by the phase change recording material in the amorphous state.
In the above-described multi-layered optical disc, according to the present invention, in which the information recording layer is multi-layered, the recording density is increased even in the three-dimensional direction corresponding to the direction of thickness of the information recording layer, thus further increasing the recording capacity.
Moreover, since the multi-layered optical disc is optically optimized, recording and/or reproduction can be made in stability and simultaneously in respective recording layers.
In an information recording medium according to the present invention, a crystallization promoting film and an enhancement film are formed at least on the side of the supporting substrate or on the opposite side to the supporting substrate of the film of the phase change material, wherein the information recording layer closest to the supporting substrate has a reflecting film allocated to an outermost side thereof towards the supporting substrate and also has a semi-transparent enhancement film allocated to an outermost side thereof opposite to the supporting substrate, and wherein the crystallization promoting film is formed of a material containing at least one of Si, SiC, Ge, GeC, Sn, SnC, Al, AlC, Ga, GaC, In, InC, chlorides and oxides thereof.
In the above information recording medium, the enhancement film is formed of a material containing at least one of ZnS, ZnSxe2x80x94SiO2, SiO2 and MgF2.
In the above information recording medium, the semi-transparent enhancement film is formed of a material containing at least one of Au, an AuCo alloy, Si, an Ag alloy, SiOx, ZnSxe2x80x94SiOx, an Auxe2x80x94SiO2 mixture and an Auxe2x80x94ZnSxe2x80x94SiO2 mixture.
In the configuration of the present invention, a crystallization promoting film and an enhancement film are formed at least on the side of the supporting substrate or on the opposite side to the supporting substrate of the film of the phase change material to constitute the information recording layers by the phase change material. Thus, the crystallization promoting film operates for crystallizing the amorphous portion, whilst the enhancement film operates for securing the light volume difference of the return light. It is noted that, since the information recording layer closest to the supporting substrate has a reflecting film allocated to an outermost side thereof towards the supporting substrate and also has a semi-transparent enhancement film allocated to an outermost side thereof opposite to the supporting substrate, the transmitted light can be effectively utilized by multi-path reflection on the information recording layer towards the supporting substrate side, where the incident light volume is minimum, thus correspondingly improving the sensitivity. Thus, if the information recording layer is multi-layered, it is possible to evade deterioration of the information recorded on the information recording layer to enable positive accessing to the lower layer side where the light volume is decreased. By forming the crystallization promoting film of a material containing at least one of Si, SiC, Ge, GeC, Sn, SnC, Al, AlC, Ga, GaC, In, InC, chlorides and oxides thereof, the amorphized phase change material can be easily melted and crystallized even if the beam spot diameter is reduced to increase the linear speed.
If, in the similar film structure, the enhancement film is formed of a material containing at least one of Au, an AuCo alloy, a SiAg allory, SiOx, ZnSxe2x80x94SiOx, an Auxe2x80x94SiO2 mixture and an Auxe2x80x94ZnSxe2x80x94SiO2 mixture, the light beam reflected by the reflecting layer can be re-reflected to increase the energy afforded to the crystallized portion which is in need of much energy for melting. Thus, if the amorphous portion and the crystallized portion co-exist, these portions can be melted evenly to re-record the desired information.
Also, by a similar film structure, the reflecting film can be fabricated from a material containing at least one of an Al alloy, BiSb, an Ag alloy, Au, an Au alloy, whereby it is possible to efficiently reflect and utilize the light beam transmitted through the information recording layer.
According to the present invention, as summarized above, a multi-layered optical disc can be furnished in which it is possible to increase the recording density in the three-dimensional direction and the recording capacity and which is superior in recording and/or reproducing characteristics.
Also, according to the present invention, both the first and second recording layers can be recorded and/or reproduced by providing the transmittance of the first recording layer of not less than 40% and by diminishing variations in the transmittance of the first recording layer at the time of recording and/or reproduction of the second recording layer.
If, when the first recording layer employs an overwritable phase change recording material, signals are previously recorded by the disc producer on the entire information recording area of the first recording layer, the transmittance of the first recording layer is rendered constant at the time of use of the disc by the user to enable stable recording and/or reproduction.
If, before and after information recording on the first recording layer, variations in the transmittance of the first recording layer are suppressed to be within the power margin of the second recording layer, information recording on the second recording layer can be performed in stability irrespective of the recording state on the first recording layer. In this case, it is unnecessary for the disc producer to pre-record the information, so that it becomes possible to suppress the production cost and the cost involved in media to raise the price competitiveness of the media.
Moreover, there is no necessity of imposing restrictions on the recording sequence for the first and second recording layers and hence the two recording layers can be recorded and/or reproduced simultaneously with the result that the transfer rate of the information for recording and/or reproduction can be twice that of the optical disc having only a sole recording layer. This enables real-time recording of the high-vision broadcast by the high transfer rate phase change optical disc.
By forming an anti-reflection film on the surface of the light transmitting layer, the recording and/or reproducing light can be focussed on the recording layer in stability without adding a mechanism of distinguishing the recording layer from the surface of the light transmitting layer to the recording and/or reproducing drive thus enabling the drive to be produced inexpensively.
Moreover, by layering the information recording layers of a phase change material, comprised of an enhancement film and a crystallization promoting film, forming the reflecting film on only the information recording layer closest to the supporting substrate and by suitably selecting the film materials of the respective information recording layers, such an optical information recording medium may be provided which is overwritable and on which recording can be made to high density.